Because as you said, its shitposting.
Also because 'for teh lulz' and for laughs are not the same thing.
But he IS right. Leto did everything for teh lulz. He didnt do it to make himself or others laugh, he did it to seem like a big badass crime boss that nobody would touch.
maybe its just harder to see comic Joker in there without Batman to tease out the best parts of his insanity.
All I saw from Leto Joker was a common street thug who decided to act like a crazy asshole to keep people on their back foot. I mean for fuck's sake he owns a nightclub.